1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus a patterning device support is used, which carries a transmissive patterning device during projection of a pattern on a substrate. This known patterning device support carries the patterning device on a top side thereof, the patterning device resting with its bottom side on the patterning device support, in particular on a number of supporting elements being arranged on the top side of the patterning device support.
It is desirable to avoid (micro)slip between the patterning device and the patterning device support during acceleration of the patterning device support, since this may lead to image deformations, and therewith to poor product quality. The acceleration in the known lithographic apparatus is typically 80-120 m/s2. In order to avoid micro slip, the known lithographic apparatus includes a clamping device to clamp the patterning device on the patterning device support. The clamping force of the clamping device provides a friction force which prevents microslip at the acceleration levels of the known lithographic apparatus.
A known clamping device is a vacuum device which creates a vacuum between the bottom side of a patterning device and the patterning device support. Since the patterning device is of the transmissive type, only a part of the surface of the patterning device may be used to apply a vacuum to. Typically, only a part of the circumferential area, i.e. close to the edge of the patterning device, may be free to apply a vacuum force.
A modified lithographic apparatus is designed with the goal to increase the throughput of the apparatus. To reach this goal, it is desirable to increase the acceleration rates of the patterning device support, for instance at least 200 m/s2, typically about 600 m/s2. However, at these acceleration rates, the risk of microslip substantially increases.
Furthermore, in the known lithographic apparatus, the vacuum device takes a considerably long time to create a vacuum level which is sufficiently low to avoid micro slip between the patterning device support and the patterning device or to take away the vacuum pressure after the projection phase to make exchange of the patterning device possible. It will be clear that during this time needed to obtain a certain desired vacuum level, the patterning device support cannot be accelerated. Therefore, the decrease of the time needed to obtain a required pressure level, will lead to an increase of the throughput of the lithographic apparatus.